1. Field of the Invention
The present invention relates to a method of an electro-optical distance measurement. In particular, a laser beam is directed onto a measured object and a measurement light beam is reflected from the object and detected by a light detector, with the distance to the object being determined by a calculation of the light propagation time between the light emitter and the light detector.
The present invention also relates to an apparatus for an electro-optical measurement including a light emitter for emitting a laser beam directed to the measured object; a light detector for detecting the measurement light beam reflected from the object, wherein the measurement light beam is transmitted thereto by a receiving optics; and a control unit that processes the signal received by the light detector, after the signal was pre-processed and subjected to an A/D conversion, for determining the measured distance based on the light propagation time between the light emitter and the light detector.
2. Description of the Prior Art
Measurement of distances up to several hundred meters with a precision of several millimeters is of great importance in many applications, in particular in the construction industry and tero-technology industry. The dynamics of the measurement systems should be high to process both extremely weak and very strong signals. Such a system makes the use of defined cross-hairs superfluous. Distance measurement from technical surfaces without the use of cross-hairs reduces the production time and, thus, reduces the production costs, and simultaneously reduces the production tolerances.
The prior art discloses numerous methods of and apparatuses for electro-optical distance measurement from technical surfaces. In most cases of measurement, preferably, a visible beam of a laser diode is directed onto a surface of a measured object, and a beam, which is reflected from a light spot on the surface from the measurement beam, is detected by a suitable detector, preferably, a photo diode. For large measurement distances and for technical surfaces with very weak surfaces, an APD-diode is primarily used as a light detector.
The advantage of the APD-diode, compared to other photo diodes, is the APD-diode's capability to amplify the photo current that it generates upon application of a light power thereto. This amplification results from a multiplication of charge carriers, in the avalanche zone, of the APD-diode, in which there exists a high electrical field strength. The field strength sharply accelerates the charge carriers, which are generated, in response to the detection of the light beam. Further charge carriers are released from the semi-conductor material of the APD-diode because of the high energy state of the generated charge carriers. The release of the additional charges carrier leads to the amplification of the photo current. A high voltage, in the reverse direction, is needed to obtain the high electrical field strength, in the avalanche zone of the diode. This voltage is within a range of 40 to 500 V. Typical amplification factors, of the photo current, are in a range of 10 to 200. These factors largely depend on the semiconductor material (Si, InGaAs), the structure of the photo diode, bias voltage, and temperature. The amplification factor of the APD-diode is largely influenced by manufacturing tolerances.
The intensity of the measuring beam of the laser diode, as a rule, is modulated, i.e., a measuring signal is superimposed on the beam, for measuring large absolute distances. In most cases, a pulsed, sinusoidal, or quasitochastic signal is used, as a measuring signal. Depending on the measurement method used, the signal propagation time from the emitter to the detector via the measured object is determined, directly or indirectly, from the signal emitted by the light emitter, and the signal reflected from the measured object and detected by the detector. In the case of direct determination, the propagation times of the light pulses are evaluated. This method is known as a light pulse propagation time method. In the case of indirect determination, the propagation time is determined based on a phase shift or correlation of the emitted and detected signals. The distance is determined from the propagation time with the speed of light being known. This method, which depends on the method of signal evaluation, is known as a phase propagation time method or a correlation method. The method, during the measurement, with which the object is continuously impinged by a light beam, is called a continuous-wave method. The continuous-wave method includes both the correlation method, in which pseudo noise modulation is used, and the phase comparison method.
A main problem of a precise distance measurement is the elimination of parasitic propagation time changes in the light emitter and/or light detector, which depend on temperature condition, manufacturing process, and aging. The measurement is calibrated with a known reference distance to reduce errors. Different calibration methods are known.
One known method, disclosed in EP 0 701 702 B1 and U.S. Pat. No. 3,992,615, uses a mechanically reversible reference track. With this method, during the first measurement step, a modulated laser beam is directed onto the measured object and, during the second, reference measurement step, the modulated laser beam is directed, e.g., via a tilted mirror or an adjustable light guide, directly to the photo detector. By subtracting measurement distances, all influences of the temperatures and aging on the components of the apparatus and the particularities caused by the manufacturing process are considered. A significant drawback of this method, is the use of movable mechanical components, which reduce the reliability and service life of the entire system.
In other measurement apparatuses of the type described, two detectors are used, a reference light detector and a main light detector, as disclosed in DE 196 43 287 A1, DE 43 28 553 A1, EP 0 610 918 B1, and DE 41 09 844 C1. With these apparatuses, a greater portion of the modulated laser beam is directed onto the measured object, with the reflected light being directed to the main light detector, and a smaller portion of the modulated laser beam being directed directly to the reference light detector. The reference light detector is constantly illuminated such that no movable mechanical change-over commutator is required. With these apparatuses, parasitic, temperature, aging, manufacture-dependent, propagation time errors associated with the light emitter are completely eliminated. The propagation time errors associated with the receiving components, however, which differ for the measurement and reference branches, remain. Paired photo diodes and/or correction tables are used, as a rule, to reduce the propagation time errors, associated with the receiving components.
Another method of the elimination of propagation time errors involves use of two light emitters and two light detectors, as disclosed in U.S. Pat. No. 4,403,857 and DE 100 06 493. In this method, a portion of the modulated light of the main light emitter is directed onto an object, from which it reaches the main light detector, as a diffused light. Another portion of the modulated light is directed directly to the reference light detector. Additionally, a portion of the modulated light of the reference light emitter is directed directly to the main light detector, while another portion is directed to the reference light detector. Similarly, with this method, no mechanical commutator is needed and all propagation time errors, in both the sender or emitter unit and in the receiving unit, are eliminated. The use of two light detectors and/or two light emitters, however, results in increased costs and a more complicated system.
In some of distance measurement apparatuses, e.g., those used in geodesy, only measurement of relatively large distances (e.g., >10 m) are of interest. In these apparatuses, the measurement and reference signals can be distinguished by a time slot, as disclosed in DE 32 16 313 C2, DE 33 22 145 A1, and EP 0 427 969 A2. Where a reference path is, e.g., 5 cm, a reference signal is detected only 0.17 ns after its emission. When a measured distance is 10 m, the detector detects the measurement signal only 67 ns after its emission. The two signals can be separated from each other because of their different detection time. Such a calibration, as a rule, is used with the light pulse propagation time method. The use of this method has some problems when small distances are measured since the detection times differ by less than 100 ps. The signal processing electronics should be able to directly distinguish these short time differences.
In the indirect phase propagation time method, a sinusoidal, intensity-modulated laser beam is used. The distance is determined by measuring the phase difference between the emitted and detected sinusoidal signals. For calibration, either one reference path, one light emitter, and two light detectors are used, as disclosed in EP 0 701 702 B1 U.S. Pat. No. 3,992,615, or two reference paths, two light emitters, and two light detectors are used, as disclosed in U.S. Pat. No. 4,403,857 and DE 100 06 493. For obtaining a high measurement accuracy, very high modulation frequencies, from 100 MHz to 1 GHz, are used. Because of the high modulation frequency, up to 1 GHz, only small distances, e.g., up to 15 cm, can be unambiguously measured. For an unambiguous determination of a distance, according to DE 100 06 493, the measurement should be effected with at least two modulation frequencies. To ensure a convenient, cost-effective, and substantially disturbance-free signal evaluation, as a rule, the high-frequency receivable signals are converted, into a lower frequency region, using non-linear signal mixing processes.
With the above-discussed method of the signal conversion, APD-diodes are used for effecting direct mixing, as disclosed in DE 196 43 287 A1 and DE 100 37 209.0. In this method, a sinusoidal signal of a local oscillator (LO), with a frequency fLO and an amplitude of more than 1V, is superimposed on a high bias voltage of the APD-diode. Thereby, the amplification factor M of the photo diode, i.e., its inner current source, is modulated by the bias voltage.
The output current of the APD diode is calculated by the following equationiAPD(t)=M(t).iphoto, o(t),
Where M(t) is a time-dependent, modulated APD-diode amplification, and iphoto, o(t) is the inner photo current generated by the detected light. The non-linear relationship between the APD-diode amplification and the inner photo current produces an intermediate frequency signal that oscillates with a frequency difference between the frequency of the local oscillator fLO and the frequency of the modulated, detected light output fMes. The frequency conversion takes place within the inner current source of the APD-diode. The high frequency components are eliminated by low-pass filtration. The output signal of the APD-diode, i.e., the intermediate frequency signal, has a relatively low frequency and, thus, can be conveniently processed. The structural dimensions of the device are typically three to four times smaller than those of conventional modulation wave modulators since the mixing process takes place within the chip of the APD-diode. As a result, parasitic interferences of the electromagnetic interference fields and of the electrical cross-talk are eliminated as much as possible. Furthermore, the noise characteristics are also improved. A reduced interference output results in reduced noise. In addition, the intermediate frequency signal, which is obtained as a result of direct mixing, has a lower frequency compared with the modulation frequency, of the detected light output, e.g., 1 kHz to 100 kHz, such that no disturbing interferences are expected. Parasitic characteristics of electronic components are also eliminated. No high-frequency components are required, in the receiving part of the circuit, except the local oscillator, since the output signal of the APD-diode lies in the intermediate frequency region. Thereby, the costs and the current consumption of the apparatus are substantially reduced. Because of the weak measuring signal, the system has only a very small, interfering, electronic noise and a very weak electrical cross-talk, e.g., less than 110 db, during the signal transmission from the laser diode emitter to the photo diode receiver. Thus, based on the above discussion it is apparent that the method of direct mixing by using an APD-diode has many advantages.
The above-described direct mixing is a heterodyne process since the LO-signal and the measurement signal have different frequencies. The intermediate signal is, thus, an A.C. signal. In a homodyne process, the LO-signal and the measurement signal have the same frequencies. In such a process, the intermediate frequency signal is a D.C. signal. A heterodyne process is primarily used for a high precision distance measurement since the IF-signals, as A.C. signals, are substantially better amplified and processed than D.C. signals. The D.C. signals are subject to a D.C.-offset, which may be greater than the original measurement signal and which is not constant. Offset or operational point fluctuation and flicker noise play a significant role in such a process. As a rule, at low frequencies, flicker noise or 1/f noise increases with the reduction of frequency and dominates. Such noise, however, is substantially eliminated at frequencies above 1 kHz. DE 44 39 298 A1 describes a homodyne process for a three-dimensional measurement of objects. In such a process, the homodyne signal mixing is effected by a two-dimensional intensity modulator. Only relative distances are of interest for such measurements. Accordingly, no ambiguity is present.
In the indirect correlation method, the light emitter is intensity-modulated with a pseudo noise signal, as disclosed in DE 42 17 423 A1 or with a timely, not equidistant pulse train, as disclosed in EP 0 786 097 B1. The emitted and detected signals are shifted in time because of the measured distance. The correlation of both signals provides for a time shift. Equidistant in time or periodical pulses, however, cannot be used since they can lead to ambiguities. In comparison, with the necessary time resolution, e.g., 10 ps for 1.5 mm of distance measurement, the times of the measurement signal changes are relatively large, e.g., 10 ns. Such necessary high accuracy is achieved by the signal correlation. The measurement band width is narrowed by appropriate correlation integrals.
With known direct light pulse propagation time methods, the measurement beam, which is emitted by a light emitter, is intensity modulated in a pulsed form. The light pulse with a width of e.g., 1 ns is reflected from the measured object and is detected by a light detector. The time between the detection of the reference and the detection of the measurement signal is determined, e.g., by a counter. Then, a next pulse is generated, and the above-described process is repeated. Primarily, the result is obtained after numerous repetitions. At the distance, e.g., of 200 m, for the sake of clarity, the repetition frequency of light pulses should be smaller than 750 kHz. As with the correlation process, this measurement process does not directly require a high time resolution of, e.g., 10 ps which would have been necessary for a single measurement with a precision of, e.g., 1.5 mm. With the available statistical jitter of the laser pulse and the actuation time points of the counter, the rough time resolution of a single measurement is successively improved by averaging the numerous results.
In the method disclosed in DE 33 22 145, the pulse propagation times are first roughly propagation times that are roughly measured first with a counter, which has, at a cycle frequency of, e.g., 1 bHz, a time resolution of only 1 ns. Such an arrangement corresponds to a distance resolution of 15 cm. For measuring the rest time, with each count, a linear voltage ramp, which stops upon detection of the measurement signal, is started anew. The height of the voltage ramp is a measure of the rest time.
In the method disclosed in DE 36 20 226 A1, pulse signals with a repetition frequency from 10 kHz to 150 kHz, which result in an unambiguous measurement after detection and amplification, are processed in a A/D converter and are continuously and timely added to each other in a parallel adder. The continuous addition successively improves the signal-to-noise ratio and the time resolution, as a result of pulse jitter.
EP 0 427 969 A2 describes a variation of the method disclosed in DE 36 20 226 A1. In the method disclosed in EP 0 427 969 A2, when a signal is very strong due to activating a differentiating member, which happens primarily during measurement of short distances, the overflow of the A/D converter is prevented, whereby the measurement accuracy is increased. Thus, for smaller distances, the measurement system is modified.
German patent DE 32 16 313 C2 discloses regulation of the light pulse power with an attenuation filter mechanically displaceable in the beam path. An example of such regulation can be found, in the above described known light pulse propagation time methods.
In the method disclosed in EP 0 610 918 B1, for a distance measurement, short pulse trains are used. After detection, a pulse train excites an electronic resonator adapted to the pulse train frequency. The resonator signal causes the laser to emit a new pulse train. The process is continuously repeated producing a pulse gyration with a predetermined gyration frequency. The distance measurement is determined from the gyration frequency.
Similarly, DE 41 09 844 C1 discloses the above-described known light pulse propagation time method. According to this method, a fiber-optic guide ring with a reference light pulse circulating in the ring is used. With each circulation, a small pulse portion is de-coupled and directed to a detector, which generates a timing signal of a counter. The counter determines the propagation time of the measurement pulse. The method also includes initiation of a reference cycle.
DE 44 39 298 A1 discloses a method of a three-dimensional measurement of objects, which is based on the above-discussed, phase comparison method with a homodyne signal mixing. In the method disclosed in DE 44 39 298 A1 in addition to the phase comparison method, the pulse propagation time method is used, wherein the object is illuminated with a light pulse. A light pulse portion, which is reflected from each point of the measured object, is mapped onto a two-dimensional detector array, e.g., CCD-array, with the aid of a receiving optics. In this way, each detector cell is associated with a certain point of the measured object. In front of the detector array, a two-dimensional optical mixer is arranged, which is also called a spatial light modulator. This light modular, e.g., a Pockets cell, functions as an optical switch. This switch becomes transparent only for a short time and, thus, only provides for passing of a light pulse portion there-through, wherein the propagation time of the pulse is correlated with the time slot of the switch. The transmitted pulse is integrated by predetermined cells of the detector array associated with corresponding points of the measured object. The time slot of the switch is successively shifted-such that, in accordance with the time slot shift, other points of the measured object are integrated. Thus, the measured object can be scanned by the time slot in pieces. The shifting of the time slot corresponds to a two-dimensional correlation or to superimposition of the received signal with the time slot. The repetition frequency of the measuring pulse and of the pulse generated by the time slot are the same. Thus, in the disclosed method, the homodyne signal mixing process is used.
DE 197 04 496 A1 and DE 198 21 974 A1 disclose advantageous embodiments of the measurement method disclosed in DE 44 39 298 A1, and according to which, certain components of the two-dimensional homodyne signal mixing circuit are pre-adjusted.
An article, entitled “Distance Measurement Using a Pulse Train Emitted from a Laser Diode,” Japanese J. of Appl. Physics, Vol. 26, No. 10, p.p. L1690 L1692, October, 1987, by K. Seta and T. Ohishi, describes a distance measurement process, in which, a measurement beam of a laser diode is modulated by a train of very short light pulses having a small duty factor. The pulse train consists of a basic frequency of 272 MHz and numerous harmonics. The pulse train reflected from the object is detected with a APD-diode. The first harmonic of the detected pulse train with a frequency of 544 MHz is converted into an IF-region of about 20 kHz by a heterodyne mixing with a sinusoidal LO-signal. Thus, the LO-signal has a frequency of 544.02 MHz. The distance measurement is effected using the first harmonic on the basis of the phase comparison method. The advantage of using the first harmonic as a measurement frequency is the elimination of the cross-talk at the basic frequency of 272 MHz and in the high measurement frequency, which is automatically obtained as a result of a so-called spiking operation resulting from the properties of the laser diode.
The advantage of the sinusoidal intensity-modulation of a laser beam using the signal mixing, according to the phase propagation time method, is that the frequency of the measurement signal is reduced, which ensures a cost-effective, convenient, and substantially disturbance-free and noise-free processing of the signal. As a result, a high accuracy is achieved. In addition, advantageously, the method permits the use of the direct mixing process. It also permits the use of economical components of the telecommunication technology. In the phase time propagation method, the same methods of signal generation and similar frequency regions are used. The drawback of the method, with a continuous process, is that only low amplitudes of the light intensity or the light output can be used to prevent damage of the eye-sight of an operator. The amplitude of the modulated output or power of the laser light should be limited to a maximum of 1 m W. Generally, the measurement accuracy depends on measurement time TMes, the amplitude of the modulated light intensity, and the measurement frequency. A standard deviation of the measurement result is determined from the following equation:                                           Δ            ⁢                                                   ⁢                          d              Phase                                =                                    Const              ⁢                                                                    3                    8                                                  ·                                  1                  π                                ·                c                            ⁢                                                1                                      P                                          LASER                      ,                      CW                                                                      ·                                  1                                                            T                      Mes                                                                      ·                                  1                                                            f                      Mes                                                                                            =                                          Const                                  1                  ⁢                                                                           ⁢                  mW                                            ·                                                3                  8                                            ·                              1                π                            ·              c              ·                              1                                                      T                    Mes                                                              ·                              1                                                      f                    Mes                                                                                      ,                            (        1        )            assuming a direct mixing with an ideal mixing efficiency is used.
From the equation (1), it follows that the standard deviation is inversely proportional to the measurement frequency fMes, to the amplitude of the laser output and to the square root of the measurement time TMes. In a complete demodulation, this amplitude corresponds to the mean laser output PLASER, CW, which as it has been discussed previously, should not exceed 1 mW to protect the operator's eye-sight.
The main advantage of the light pulse propagation time method is the possibility to use more intensive light pulses while insuring protection of an operator's eye-sight. For short light pulses, less than 18 ns, to ensure the eye-sight protection, the mean light output PLASER, cw should not exceed 1 mW. For an operation with a reliable eye-sight protection, with the maximum possible pulse light output PLASER, CW, the following equation applies:                               P                      LASER            ,            M                          =                                            P                              LASER                ,                CW                                                    η              Duty                                =                                    1              ⁢                                                           ⁢              mW                                      η              Duty                                                          (                  2          ⁢          a                )            with a standard deviation of the measurement result:                                           Δ            ⁢                                                   ⁢                          d              pulses                                =                                    Const              ·              c              ·                              1                                  P                                      LASER                    ,                                          1                      ⁢                      M                                                                                  ·                              1                                                      2                    ⁢                    π                                                              ·                              1                                                                            T                      Mes                                        ⁢                                          η                      Duty                                                                                  ·                              t                Rise                                      =                                                            Const                                      1                    ⁢                                                                                   ⁢                    mW                                                  ·                c                ·                                  1                                                            2                      ⁢                      π                                                                      ·                                                      η                    Duty                                                  ·                                  1                                                            T                      Mes                                                                      ·                                  t                  Rise                                            ≅                                                Const                                      1                    ⁢                                                                                   ⁢                    mW                                                  ·                c                ·                                  1                                                            2                      ⁢                                              π                        3                                                                                            ·                                                      η                    Duty                                                  ·                                  1                                      f                    L                                                  ·                                  1                                                            T                      Mes                                                                                                          ,                            (                  2          ⁢          b                )            where tRise designates the rise time of the detected pulse of L≡1/(πtRise) the 3-dB limited frequency of the system, and ηDuty, the duty factor of the pulse train. The distance measurement accuracy or precision is proportional to the square root of the duty factor ηDuty of the pulse train and is inversely proportional to the limited frequency fL of the system. The advantage, which flows from equation (2b), is an increase of the light pulse output by a factor 1/ηDuty, which carries more weight than the reduction of the effective measuring time TEff=TMes·ηDuty by a factor ηDuty. This advantage permits an improvement in the signal-to-noise ratio. In comparison with a continuous process, the measurement error is reduced, with the use of the light pulse propagation time method, by a factor:                     Γ        =                                            Δ              ⁢                                                           ⁢                              d                pulses                                                    Δ              ⁢                                                           ⁢                              d                Phase                                              =                                                    8                3                                      ·                          1                                                2                  ⁢                  π                                                      ·                                          η                Duty                                      ·                                          f                Mes                                            f                L                                                                        (        3        )            For a pulse length of, e.g., 2tRise=1 ns and a repetition frequency of, e.g., 750 kHz, the duty factor ηDuty=1/1333, and the limiting frequency of the system fL=637 MHz. When the limiting frequency fL for the pulse propagation time measurement and the measurement frequency for the phase propagation time measurement fmess are the same, ideally, the measurement error is reduced by a factor Γ=1/55. The essential drawback of the light pulse propagation time method is the necessity to use gigahertz counters or more rapid scanning circuits, e.g., more than 100 mega samples/sec, which is connected with higher costs resulting, partially, from components that are not readily available. In addition, because the duty factor is very small, which is necessary to obtain unambiguous results, a very high optical pulse output, e.g., several watts, is required. This output can be achieved only with special infrared laser diodes, which are expensive and not readily available. On the one hand, laser diodes emitting non-visible laser beam signal mixing in a region of low repetition frequencies is rather difficult and, on the other hand, such diodes do not provide any significant advantage because of low repetition frequency.
In comparison to the phase propagation time method, the correlation method, in which short, non-periodic, light pulses are used, permits the use of higher optical signal outputs while insuring eye-sight protection. As a result, at the same signal-noise-gap, the total measurement time is reduced, and the effective measurement time decreases due to the pulsed operation. The distance measurement accuracy of the light pulse correlation method is determined by the following equation:                               Δ          ⁢                                           ⁢                      d            Corr                          =                              Const            ·            c            ·                          1                                                2                  ⁢                  π                                                      ·                          1                              P                                  LASER                  ,                  CW                                                      ·                                          T                Eff                                            T                Mes                                      ·                                          t                Rise                                                              T                  Eff                                                              ≅                                    Const                              1                ⁢                                                                   ⁢                mW                                      ·            c            ·                          1                                                2                  ⁢                                      π                    3                                                                        ·                          1                              f                L                                      ·                                                            T                  Eff                                                            T                Mes                                                                        (        4        )            
The distance measurement accuracy is inversely proportional to the laser signal output PLASER,CW. TMes/TEff and proportional to the effective measurement time Teff and to the reciprocal square root thereof. The maximum allowable laser signal output again is inversely proportional to the effective measurement time. The effective measurement time is determined as a total duration of the measurement pulse detected during the total measurement time TMes. The signal-to-noise ratio of the light pulse correlation method lies between the signal-to-noise ratio of the phase propagation time method and the signal-to-noise ratio of the pulse propagation time method. The correlation method, in which a pseudo noise modulation is used, as the continuous process, does not have an improved signal-to-noise ratio. As with the light pulse propagation time method, the drawback of the correlation process is the necessity to use rapid scanning circuit, e.g., more than 100 mega samples/sec, which is associated with high costs of component and high costs of signal generation and signal processing. Signal mixing in the low frequency region was not contemplated and would not have been advantageous since non-periodical signals are used.
The object of the present invention is to provide a method of and an apparatus for an electro-optical measurement of comparatively large distances and a measurement from weak reflecting objects without the use of cooperating cross-hairs.
Another object of the present invention is to provide a method of and an apparatus for an electro-optical measurement of comparatively large distances and a measurement from weakly reflecting objects, which are highly reliable and have high measurement precision and, which are inexpensive, while ensuring a high protection of the human eye from being damaged by an electro-optical beam, in particular, a laser beam.